Conventionally, an uninterruptible power supply apparatus has been widely used as a power supply apparatus for stably supplying AC electric power to an important load such as a computer system. As described in Japanese Patent Laying-Open No. 2001-61238 (PTL 1), for example, the uninterruptible power supply apparatus generally includes a converter converting first AC electric power from a commercial AC power supply to DC electric power, a power storage device storing the DC electric power, and an inverter converting the DC electric power supplied from the converter or the power storage device to second AC electric power having a commercial frequency and supplying the second AC electric power to a load.
In a normal state in which the first AC electric power is supplied from the commercial AC power supply, the converter converts the first AC electric power to the DC electric power, and supplies the DC electric power to the inverter while charging the power storage device. The inverter converts the DC electric power to the second AC electric power and supplies the second AC electric power to the load. When a power failure occurs at the commercial AC power supply, the DC electric power is supplied from the power storage device to the inverter and the inverter continues supplying the second AC electric power to the load.